Operation of tubular solid oxide fuel cells in a fuel cell generator is well known in the art, and taught, for example, in U.S. Pat. Nos. 4,395,468, 4,664,986 AND 5,573,867 (Isenberg, Draper et al., and Zafred, et al., respectively). There, the generator is divided into an oxidant inlet chamber and a fuel inlet chamber, separated by a combustion zone or air pre-heating chamber. Tubular solid oxide electrolyte fuel cells extend from the combustion zone or air pre-heating chamber and through the fuel chamber. The term "tubular" as used here is defined as meaning circular, as well as flattened configurations containing a plurality of interior oxidant passages, as taught in U.S. Pat. No. 4,874,678 (Reichner). The tubular solid oxide fuel cells have a closed end in the fuel chamber and an open end within the combustion zone or air pre-heating chamber, where depleted oxidant passes out of the fuel cell open end to combust with depleted fuel, to pre-heat oxidant feed tubes and feed oxidant passing through those feed tubes. The fuel cells have an outer electrode contacting flowing fuel, the "fuel electrode" anode, and an inner support electrode contacting flowing oxidant, usually air, the "air electrode" cathode, separated by a solid oxide ceramic ionically conductive electrolyte, and operate from about 900.degree. C. to 1300.degree. C. The air electrode is usually made of a doped-LaMnO.sub.3.
Solid oxide fuel cell (SOFC) systems currently being developed for power generation applications, offer high efficiency, negligible stack pollution and ease of operation by utilization of many types of fuels. Several SOFC power generation systems capable of operating on gaseous and liquid hydrocarbon fuels have been fabricated and field tested to evaluate the performance and long term stability of cells and system components. The integration and operation of SOFC systems with gas turbines/generators in a pressurized mode offers the potential of more efficient operation. Operating the SOFC generator under pressurized conditions is beneficial in the reduction of cathode-side polarization losses, however cell operation under pressure may increase the risks involved in upset and transient conditions.
If balanced pressure is not maintained on both anode and cathode sides of the cell, gas flow will occur, between sides, releasing oxygen to the anode side or fuel gas to the cathode on the inside of the cell. The introduction of fuel gas to the cathode causes reduction of the doped-LaMnO.sub.3 with a concurrent volume change that mechanically stresses the material which can lead to cracking and cell failure. Recognizing that a fuel cell generator can have hundreds of separate fuel cells, making up most of the generator, this can be a serious problem. The main means to control such cracking problem was to attempt maintenance of standard operating conditions, whereby fuel gas was isolated from the inside of the cell. This, however, does not solve problems during upset conditions when a sudden depressurization event occurs. Thus, there is a need for a permanent solution to fuel incursion into the open end of the fuel cell to degrade the interior air electrode. The main object of the invention is to protect the interior air electrode from fuel contact and degradation during operation and under upset conditions by providing a thermally shock resistant sleeve.